This system relates to ink jet printing, and more particularly to the collection of unused ink during ink jet printing.
Ink jet printing involves the projection of ink from a nozzle or tube onto a recording medium. Ink jet systems fall into two broad categories in terms of the percentage of the ink drops formed by the system which finally impinge on the recording medium. One class of ink jet devices, the so-called "drop-on-demand" systems, forms ink drops in response to a video signal, all of which drops are used in recording. Two examples of this approach are disclosed in U.S. Pat. Nos. 2,143,376 and 3,341,859. In these systems, a conductive bar is placed behind the recording medium and ink is drawn from the nozzle in response to an applied voltage pulse of opposite polarity.
Those in the second category, the "continuous flow" systems, employ projection of ink from a vibrating nozzle. The ink drops pass through a charging tunnel where they are selectively charged in accordance with a video signal. Further along the path a set of deflecting plates or electrodes functions to deflect the charged drops in response to an applied potential difference. These systems vary in whether the charged drops are finally used in printing, but all share the characteristic that only a small percentage of the ink drops projected from the nozzle finally find their way to the receptor.
In any continuous flow system, therefore, the unused ink must be efficiently collected and removed from the ink stream area to insure continuous, unobstructed flow of ink, and effective deflection of charged ink drops. This has posed design problems for such systems. One approach, which underlies the collection system of the present invention, has been to deflect the charged drops onto a knife edge of a collection electrode (the companion electrode of which is denoted the deflection electrode), where they are drawn by a vacuum system through a porous, stainless steel wall of the collection electrode, and away from the ink flow area. Apparatus utilizing this collection approach is disclosed in Hertz, U.S. Pat. No. 3,916,421.
A collection system of this description is capable of effectively removing only those ink drops which strike the knife edge. Stray ink drops which are prematurely deflected and never strike the knife edge, as well as ink drops which bounce off the knife edge, adhere to and dry upon the surfaces of the collection and deflection electrodes. The vacuum suction system is incapable of removing many of these stray ink drops. The utilization of a second vacuum system to draw ink drops through the deflection electrode reduces the number of ink drops which adhere to the surface of the deflection electrode. This additional vacuum element, however, has no effect on the drops which adhere to the surface of the collection electrode between the knife edge and the charging tunnel.
The accumulation of dried ink drops along the surface of the collection electrode interferes with the flow of ink drops required for continuous ink jet printing. A buildup of dried ink drops along the surface of the collection electrode, particularly at the point where the channel between the collection and deflection electrodes is narrowed by the presence of a deflection step (a narrower area of enhanced deflection), will physically obstruct the flow of ink drops through the electrode assembly. The accumulation of ink drops along the surface of the collection and deflection electrodes therefore prevents effective deflection of undesired ink drops. The narrowed distance between the two electrodes encourages an electric arc to occur between them, producing a momentary loss of voltage in both electrodes. Intermittent loss of voltage prevents continuous deflection and frustrates effective printing of the desired code. Moreover, ink drops dried upon the surfaces of the collection and deflection electrodes may accumulate to the point where an actual bridge is formed between the two electrodes, thereby causing the electrodes to short, completely halting the deflection process.
The problems caused by the drying of ink drops on the electrode structure are especially serious when a solvent based ink is being employed. For printing on paper and other porous surfaces, it is possible to use a water based ink with drying inhibitors. For printing on surfaces such as metal, however, it is necessary to use a solvent based ink. This type of ink is likely to dry in undesirable locations unless its solvent content is maintained.
Accordingly, it is a primary object of the invention to achieve a continuous, unobstructed flow of ink drops through an electrode structure during ink jet printing. A subsidiary object is to avoid accumulation of dried ink drops on the surface of the collection electrode. A related object is to collect and remove stray ink drops before they dry upon the surface of the collection electrode.
Another object of the invention is to effect continuous deflection of ink drops not intended for printing. A related object is to avoid sporadic losses in voltage due to arcing.
A further object of the invention is the avoidance of ink drop bridging between the collection and deflection electrodes. It is a related object to prevent stray ink drops from adhering to the deflection electrode.
Yet another object is the facilitation of ink jet printing with solvent based inks.